1. Field of the Invention
This invention relates to nacelle and bypass duct construction for wing-mounted gas turbofan aircraft engines.
2. Summary of the Prior Art
It is well known that lifting forces are produced by an aircraft wing during flight as a result of a pressure differences acting over the wing platform. As the wing passes through a volume of air, relatively high air pressure is developed below the wing and relatively low air pressure is developed above the wing. In general, the greater the difference in pressure between the upper surface and lower surface of the wing, the greater the lift produced by the wing. It is also well known that as the airplane is more steeply angled, the angle of attack of the wing is increased, and the pressure differences and lift are correspondingly increased. Unfortunately, an increase in angle of attack also has a corresponding effect on aerodynamic drag produced by the wing. Because the angle of attack of the wing is increased to produce more lift, the wing projects a greater frontal area causing the increase in drag.
When an aircraft is traveling at subsonic speeds, an engine that is positioned beneath the aircraft wing causes the local wing underside pressures to be lower than what they would be under the same wing without the engine. This localized lowering of the underside pressure results in a decreased pressure differential and reduces the wing lift for a given angle of attack. Since a given aircraft requires a fixed amount of lift to maintain altitude at a given cruise velocity, the wing angle of attack must be increased to regain that amount of lift which is lost due to the presence of the engine nacelle. As expected, this increase in the angle of attack required to offset the lift loss caused by the engine results in another increase in aerodynamic drag. Those skilled in the art commonly refer to this drag produced by the presence of the nacelle under the wing as "interference drag."
Analysis of interference drag has revealed that different engine nacelle shapes may have similar or identical isolated drag by themselves in an airstream, but can have very different effects on a wing pressure distribution, and thus create widely differing amounts of interference drag. Further analysis has been directed at understanding these differences and the causes of this interference drag. The results of this analysis indicate that efforts should be directed towards minimizing the effect of engine fan exhaust systems on wing pressure distribution for the purpose of reducing interference drag.